1. Field of the Invention
The present invention relates generally to analog switches, and more specifically, to metal-oxide semiconductor analog switches.
2. Background Information
Flatness of the on-resistance of a metal-oxide semiconductor ("MOS") analog switch is an important parameter in many applications. The conventional way to achieve reasonable flatness over a specified signal range is to have an N-channel MOS transistor and a P-channel MOS transistor connected in parallel. FIG. 1 is a circuit diagram of such a prior art analog switch. As shown, the switch consists of an N-channel MOS device N1 connected in parallel to a P-channel MOS device P1. More specifically, the respective sources and drains of the devices N1 and P1 are coupled together, with the input terminal IN coupled to the sources of the devices N1 and P1, and the output terminal OUT coupled to the drains of the devices N1 and P1. A control terminal CONTROL is coupled to the gate of device N1, and to the gate of device P1 through an inverter I1.
When a signal on the CONTROL terminal is low, both devices N1 and P1 are off, and thus the analog switch is off, prohibiting a signal on the IN terminal to be passed to the OUT terminal. On the other hand, when the CONTROL terminal is high, both devices N1 and P1 are on, and thus the analog switch is on to couple a signal on the IN terminal to the OUT terminal. For a change in input voltage in either direction, the on-resistance variation of the N-channel device N1 is compensated, to a certain extent, by the on-resistance variation of the P-channel device P1 in the opposite direction.
However, this type of analog switch has several drawbacks. One main drawback is that the on-resistances of the N-channel and P-channel devices N1 and P1 do not track each other exactly. Therefore, the on-resistance still shows a variation with the input voltage level, becoming worse as the signal approaches either supply rail. Another drawback to using an N-channel/P-channel combination is that the size of the P-channel device is 3 to 4 times the size of the N-channel device because of the lower hole mobility of silicon. This results in a larger parasitic capacitance and consequently, a reduction in bandwidth.
Accordingly, there is a need in the technology for an analog switch which overcomes the aforementioned drawbacks.